Pseudo-Jahn-Teller optical absorption of isolated nitrogen in diamond

Using optical absorption measurements of synthetic diamond under uniaxial stress at room temperature two bands were identified with peaks at 3.9 eV and at 3.3 eV, being polarised perpendicular and parallel, respectively, with respect to a trigonal (111) axis. They are attributed to transitions between the ground and first excited states derived from a pseudo-Jahn-Teller model of the trigonally distorted N donor in diamond. The model yields a ground-state stabilisation energy of 0.87 eV, considering only T 2 mode coupling. The limitations of the model are discussed. The N band at 4.5 eV is shown to be polarised parallel to (111). It can possibly be assigned to a charge transfer transition from the valence band into a nitrogen donor level.